This invention relates generally to the field of excavating hard, massive rock, and more specifically to a cutterhead, hydraulic dredge, and method for excavating said rock underwater, whereby the configuration of the cutter and blade placement equidistant from the shaft assist in causing the application of proper Kinetic Energy to rock to assure its disintegration without premature failure of cutting blades. Previously, the industry has always relied upon the shearing strength of a cutter on the soil. However, rock does not shear, it must be shattered. The proper combination of cutting force and tooth velocity achieved in the present invention roughly doubles the capacity of a cutter and halves the unit cost of dredging a cubic yard.
For hundreds of years, Man has used various methods to dredge his shipping channels. He progressed from the hand-held shovel to mechanized buckets such as the dragline, clamshell, dipper and backhoe. He also developed the hydraulic dredge which utilized an excavator called the cutter or cutterhead. This tool excavated underwater soil and directed it to a high velocity stream of water entering the pumping system, which was then sent as a slurry of water and solids to the disposal area via pipeline.
Dredges, whether mechanical or hydraulic, have historically been limited in the material they could excavate. Massive, hard rock has low elasticity and must be shattered, not sheared. It was initially considered impossible to dredge, and the mechanical or bucket dredge is still severely limited on hard rock. However, the hydraulic dredge has progressed in its development of rock cutters, and in recent decades, has had some success in dredging rock, although generally accompanied by problems and sometimes failure, including project abandonment. In contrast, the present invention overcomes such problems for successful shattering of massive hard rock.
Mechanical dredges with various types of bucket excavators have attempted to dig hard rock by forcing the bucket into the rock in an effort to shear it. Unless said rock was soft, these efforts inevitably failed, regardless of the unit pressure brought to bear. Massive, friable rock responds poorly to attempts to shear it, requiring instead shattering by impact analogous to the jack hammer on concrete. Dredge operators soon learned that the impacts of the cutterhead teeth of the hydraulic dredge on rock were more effective than mechanical buckets, and efforts were made to further improve the cutter's effectiveness on rock. FIG. 1 shows the current state of the art of a rock cutterhead, which incorporates hardened, quickly-changed teeth mounted in adaptors on each of the several blades of the cutter. It is the teeth mounted in adaptors that precede the blades and protect them from contact with solid rock so that such contact seldom occurs in the absence of tooth failure. The pictured cutter is approximately ten feet in diameter, driven at the back ring by 4000 hp at approximately 30 RPM. It is a massive, complex, and expensive device. However, with its graceful blade arcs and symmetrical teeth it is a thing of beauty and a formidable tool, but its design is flawed as while it succeeded in digging hard rock, but it also suffered numerous failures in teeth and adaptors, greatly increasing downtime and unit costs, and decreasing production rate.
Cutter horsepower and cutting force are not the most significant factors in rock dredging. Rock must be shattered by impact, as measured by the Kinetic Energy of the cutter teeth in units of ft-lbs. Rock dredging experience along the U.S. east coast discloses that the limestone can be excavated by cutters with a Kinetic Energy of at least 500,000 ft-lbs using nominal cutter size, but more accurately for present invention purposes. Kinetic Energy of approximately 611,000 ft-lbs is required at the tip of each cutter tooth. Successful rock dredging, though difficult, can be accomplished by a sturdy pinned tooth cutter where each tooth exceeds the necessary Kinetic Energy to shatter rock of 611,000 ft-lbs, and whereby the total cutter force is concentrated on each tooth, as in the structure of the present invention. The prior art cutter shown in FIG. 1 has six blades (or arms) with eight teeth on each blade. Note that as the blades curve into the drive shaft hub at the closed end of the cutter, there are hardened teeth to shield the softer blades from direct contact with the rock. These teeth are necessarily closer to the cutter's rotational axis, meaning that while they rotate at the same RPM as the outer teeth, they run at a slower peripheral velocity, reducing their Kinetic Energy (KE) as expressed by the classic equation: KE=MV2/2g, where M=total cutter force in pounds; V=tooth tip velocity in ft/sec; and g=the acceleration of gravity (32.2 ft/sec/sec). Since KE varies as the square of tooth velocity, the KE of a tooth closer to the shaft is reduced significantly, and said tooth invariably encounters rock that is not shattered by the low velocity and inadequate KE. This leaves intact rock to resist the force of said tooth, causing failure of said tooth (and/or adaptor) which can be exposed to most, if not the total cutting force. To prevent failure, it is necessary that each tooth have the required Kinetic Energy to shatter rock upon impact. Excessive horsepower and its proportional cutting force can be detrimental without adequate tooth velocity, causing greater tooth failure than a lesser cutting force. Horsepower should not be confused with Kinetic Energy or work. While work is expressed in ft-lbs with one ft-lb defined as the energy to raise one pound through one foot of elevation, horsepower is the rate of doing work or expending energy in ft-lbs/minute, with one horsepower equal to 33.000 ft-lbs/minute. Further, while as mentioned above the Kinetic Energy varies as the square of tooth velocity, the horsepower (HP) in ft-lbs/minute of a cutter of a given diameter varies directly with the total cutting force (M in lbs) and RPM. Thus. HP=M×RPM×D×Pi/33,000, where D is the cutter diameter. When horsepower is defined in terms of torque (T), with T=M×D/2 as measured in ft-lbs, the formula for horsepower becomes HP=T×RPM/5252. Thus, in the HP equation, if the cutting force M is reduced by half and the horsepower or rate of doing work is to remain the same, the velocity of the cutters must be doubled. However, in the Kinetic Energy equation, it can be seen that the cutting force M is reduced by half and the cutter velocity is doubled, since V is squared to 4, twice the Kinetic Energy is netted and results in increasing the capability of the existing drive. Reduced horsepower HP and cutting force M results in lighter, less expensive equipment, such as the present invention, while the higher tooth velocity used increases the capability of the cutter, a win—win situation.